nitro star 15fe idle problems!

Hi, can any one tell me what might be stoping my Nitrostar 15fe from idleing properly, it will run but seems to get very hot and will sall as soon as the reves start to drop to a resonable level. i've tried retuning, made sure the fule tank is sealing, and checked the head and exahust seal. it hasn't got any shims in the head, not sure if this makes a difference? oh and tried new plugs. any help would be great.
Matt